1. Field of the Invention
The present invention relates to stents, and more particularly, to stent grafts having an expandable web structure configured to provide enhanced embolic protection and reduce restenosis and thrombus formation.
2. Background of the Invention
Stents are commonly indicated for a variety of intravascular and non-vascular applications, including restoration and/or maintenance of patency within a patient's vessel. Stents are also used to reduce restenosis of a blood vessel post-dilation, thereby ensuring adequate blood flow through the vessel. Previously known stents are formed of a cell or mesh structure, having apertures through which endothelial cells migrate rapidly. These endothelial cells form a smooth coating over the stent that limits interaction between the stent and blood flowing through the vessel, thereby minimizing restenosis and thrombus formation.
In many applications, in addition to maintenance of vessel patency and limitation of restenosis, protection against release of embolic material from the walls of the vessel is desired. Emboli released into the bloodstream flow downstream, where they may occlude flow and cause death, stroke, or other permanent injury to the patient. The apertures between adjoining cells of previously known stents may provide an avenue for such embolic release, depending upon the application.
In addition to embolic protection, a smooth surface, i.e. a substantially continuous surface lacking apertures, may be desired to permit unencumbered recrossability with guide wires, balloon catheters, etc., into the lumen of the stent, for example, to compress stenosis or restenosis and open the lumen, to resize the stent to accommodate vascular geometry changes, etc. Further, equalization of forces applied by or to the stent may be desired to reduce a risk of the stent causing vessel dissection. Due to the apertures, previously known stents may provide only limited embolic protection, recrossability, and force distribution in some applications.
A covered stent, or a stent graft, comprises a stent that is at least partially externally-covered, internally-lined, or sintered with a biocompatible material that is impermeable to stenotic emboli. Common covering materials include biocompatible polymers, such as polyethylene terephthalate (PETP or “Dacron”) or expanded polytetrafluoroethylene (ePTFE or “Teflon”). Stent grafts may be either balloon-expandable or self-expanding. Balloon-expandable systems may be expanded to an optimal diameter in-vivo that corresponds to the internal profile of the vessel. Upon compression, self-expanding embodiments characteristically return in a resilient fashion to their unstressed deployed configurations and are thus preferred for use in tortuous anatomy and in vessels that undergo temporary deformation.
A stent graft provides embolic protection by sealing emboli against a vessel wall and excluding the emboli from blood flow through the vessel. Additionally, since the biocompatible material of a stent graft closely tracks the profile of the stent, forces applied by and to an impinging vessel wall are distributed over a larger surface area of the stent, i.e. the force is not just applied at discrete points by “struts” located between apertures of the stent. Rather, the biocompatible material also carries the load and distributes it over the surface of the stent. Furthermore, stent grafts provide a smooth surface that allows improved or unencumbered recrossability into the lumen of the graft, especially when the biocompatible material lines the interior of, or is sintered into, the stent.
While the biocompatible materials used in stent grafts are impermeable to, and provide protection against, embolic release, they typically do not allow rapid endothelialization, because they also are impermeable or substantially impermeable to ingrowth of endothelial cells (i.e. have pores smaller than about 30 μm) that form the protective intima layer of blood vessels. These cells must migrate from the open ends of a stent graft into the interior of the stent. Migration occurs through blood flow and through the scaffold provided by the graft. Such migration is slow and may take a period of months, as opposed to the period of days to weeks required by bare (i.e. non-covered) stents.
In the interim, thrombus may form within the lumen of the graft, with potentially dire consequences. As a further drawback, migration of the endothelium through the open ends of a graft may leave the endothelial coating incomplete, i.e. it does not span a mid-portion of the graft. In addition, the endothelial layer is often thicker and more irregular than the endothelialization observed with bare stents, enhancing the risk of restenosis and thrombus formation.
Porous covered stents also are known. For example, U.S. Pat. No. 5,769,884 to Solovay describes a covered stent having porous regions near the end of the stent, wherein the pores are sized to allow tissue ingrowth and endothelialization. The middle region of the stent is described as being much less porous or non-porous, to encapsulate damaged or diseased tissue and inhibit tissue ingrowth.
The Solovay device is believed to have several drawbacks. First, the end regions of the stent are described as having a preferred pore diameter as large as 120 μm. Pore diameters greater than about 100 μm may provide inadequate embolic protection; thus, if the end regions compress a stenosis, hazardous embolization may result. Further, since the middle region of the stent is adapted to inhibit tissue ingrowth, endothelial cells must migrate into the middle region of the stent from the end regions and from blood flow. As discussed previously, such migration is slow and provides an inferior endothelial layer.
An additional drawback to previously known devices is that many are not configured for use at a vessel bifurcation. A bare stent placed across a vessel side branch is expected to disrupt flow into the side branch and create turbulence that may lead to thrombus formation. Conversely, placement of a non-porous covered stent/stent graft across the bifurcation is expected to permanently exclude the side branch from blood flow, because such grafts are substantially impermeable to blood.
In view of the drawbacks associated with previously known stents and stent grafts, it would be desirable to provide apparatus and methods for stenting that overcome the drawbacks of previously known devices.
It further would be desirable to provide methods and apparatus that reduce the risk of embolic release, while also reducing the risk of restenosis and thrombus formation.
It also would be desirable to provide apparatus and methods for stenting that allow improved recrossability into the lumen of the apparatus.
It would be desirable to provide apparatus and methods for stenting that distribute forces applied by or to the apparatus.
It still further would be desirable to provide apparatus and methods suitable for use in bifurcated vessels.